Device for determining the transparency of the atmosphere

ABSTRACT

Device for determining the transparency of the atmosphere between two points comprising one light transmitter and one receiver respectively on the two points, optic fibers connecting the corresponding transmitters and receivers, means to separate the data coming through the atmosphere and through the fibers and a data processing device connected to said means.

United States Patent 11 1 Teboul et al.

1 1 July 17, 1973 1 DEVICE FOR DETERMINING THE 3,506,360 4/1970 Albert 356/2011 x TRANSPARENCY OF THE ATMOSPHERE 3,524,707 8/1970 Hansen 1 356/207 X 3,563,661 2/1971 Charlson et a1 1 356/207 X [75] Inventors: James Remy Albert Teboul, 3,314,327 4/1967 Killpatrick et al., 356/176 X Montrouge, Hauts-de-seine; 3,458,260 7/1969 Owens et al. 356/1 15 Andr Ren BaudmAmuei]g 3,593,055 7/1971 Geusic et al. 250/71 R X 3,493,304 2/1970 Rovner 1 1 356/103 g t 13 3,340,764 9/1967 Bergson 356/212 x 0 fame 3,544,224 12/1970 Fried] 356/201 x 2,877,453 3/1959 Mendenhall lrw 356/103 X [731 Assignee: Compagnie Des Compteurs, Mont- 3,396,387 8/1968 Grant, Jr 250/218 X rouge, Hauts-de-Seim, France 3,431,423 3/1969 Keller 356/103 X 3,462,608 8/1969 Weston et a1 356/207 X [22] Filed. Sept. 3, 1969 3,506,359 4/1970 Burke, Jr. et al. 356/201 X 1 pp No 889 877 3,553,462 1/1971 Johnson 356/103 X OTHER PUBLICATIONS Weissber er Ph s. Meth. of Or Chem. lntersci- [30] Forelgn Application Prlorlty Data g 1 g 1 Se t 5 1968 France 68165207 ence Pub.; N.Y.; 1960; pp. 2,125-2,128. 3 I968 Framm 68179985 Ash eta1.,Analyt. Chem, V0|.-43,N0. 1,166. 1971.

' 6909279 21 :22: 68167867 Primary Examiner-Ronald L. Wnbcrt p Assistant Examiner-F. L. Evans 52 us. c1... 3561201, 250/218, 250/227, Parker 356/204, 356/206 51 1111.0. ..G01n 21/26 1 I ABSTRACT [58] Field of Search 356/36, 72, 102-104, Device for determining the transparency of the atmo- 356/176-177, 201-208, 114-117; 250/218, 227 sphere between two points comprisingone light transmitter and one receiver respectively on the two points, [56] References Cited optic fibers connecting the corresponding transmitters UNITED STATES PATENTS and receivers, means to separate the data coming 3 437 820 M1969 Thom Sol-l a a] 250,218 X through the atmosphere and through the fibers and 21 3447370 6/1969 Tanznfan .QII IIII 356/207 x data Processing device cmmected Said means 3,535,533 10/1970 Guidi 356 201 x 8 Claims, 7 Drawing Figures 4 7a 1:1 361 6b 3b 2 e r L M mm (P1113114) Sap-rabi- 10a (ran-"H3 1 mu/11) Ob 11 2 $13 12 LA] LE! 19] L21 d 1,, P(h ln|3 ewzull' 61 1; '16

FIG -1 PATENTED JUL 1 mm sum 1 or 6 8 AmphFmr PATENTEDJUL 171975 SHEET 2 [If 6 PATENn-jn JUL mars sum 3 m 5 Q? h J DEVICE FOR DETERMINING THE TRANSPARENCY OF THE ATMOSPHERE Numerous devices are known for measuring the dimming or absorption of light due to the diffusion of water particles contained in the atmosphere and thus determining the transparency thereof. This measuring has considerable interest in giving a measure of the visibility through fog of objects or radiant sources such as ground lights on airfield runways.

Most of the known devices are based on the measuring of the current supplied by an electric photosensitive member energized by a suitable light source through a given path in the atmosphere, to be studied. The value of this current characterizes, under known and stable geometric conditions, the fraction of emitted light, or more generally radiation which reaches the receiver as a function of the absorption due to diffusion.

The usual parameter employed is the transmissive power of the atmosphere, which is the percentage of radiation transmitted, the value 100 percent being at tributed to the quantity of radiation received by the receiver, under the same geometric conditions, when the atmosphere is perfectly pure.

It will be understood that the measurement can be erroneous if the radiant source is not of invariable intensity and if the photosensitive receiver does not have an invariable sensitiveness, actually the variations in time and variations according to external phenomena which affect the power radiated by the source, as well as the sensitiveness of the receiver, cause serious errors in a device of this kind.

To overcome such drawbacks, it is usual to stabilize the voltage of the power generator which supplies the radiant source. Unfortunately, a remedy of this kind is ineffective on the evolution of the output of the source itself. Also, the stabilizing of the sensitiveness of the receiver is still more difficult to obtain.

Most of the known appliances are based upon the principle of automatic compensation by means of a special optical path provided with a reference transmission coefficient. It is then necessary that this reference path be periodically substituted, between the light source and the photo-sensitive receiver, to the atmospheric path. Owing to the length of this atmospheric path which is considered as a sample of the atmosphere and is generally in the region of 50 meters, this method only applies to an atmospheric path comprising a return path after reflection on a mirror between the source and the receiver, then close to each other. This is disadvantageous, because in the range of transmissive powers of the atmosphere that the appliance can measure, it divides by two the length of the atmosphere sample. Moreover, the periodic substitution of one optical path for another, can only be carried out conveniently by mechanical means synchronized shutters, revolving mirrors, etc.. This type of equipment generally requires regular maintenance for a permanent service.

The present invention thus has for its object the pro vision of an entirely static device, which is not disturbed by variations in the transmission of the source, as well as in the sensitiveness of the receivers.

The present device for measuring the transmissive power of the atmosphere on a given path, is characterized in that it comprises at each end of said path, an optico-electronic transmitter-receiver, each transmitter sending light signals to its associated receiver and towards a distant receiver, means to separate the signals received by a receiver from its associated transmitter and distant transmitter, and means to deduce there from a magnitude proportional to the transmittive power by carrying out the ratio of the products of the values of homologous signals supplied by each of the receivers,

Moreover, this device also affords the advantage of preserving on the length of the atmosphere sample, the maximum value permitted by the equipment. Moreover measurement may be made independent on the gain variations of electric signal amplifiers, generally associated with photosensitive receivers used in said opticoelectronic transmitter-receiver.

Lastly, since the invention by calling upon the help of two devices of identical construction, the case of a defect in any of the constitutive members of one device, such as light source, photosensitive receiver, amplifier, etc still provides a measurement which is actually of lesser accuracy but enables the avoidance of stopping the service while the defect is being repaired.

Other provisions of the invention enable elimination of the influence of atmospheric interfering retrodiffusions, either by using reference paths, or by utiliz ing polarized light.

The invention will be better understood by referring to the following description and accompanying drawings, given by way of non-restrictive example In these drawings FIG. 1 shows diagrammatically a first embodiment of the invention.

FIG. 2 shows a transmitter-receiver unit.

FIG. 3 shows, diagrammatically, a variant of embodiment.

FIG. 4 is an explanatory diagram.

FIG. 5 shows diagrammatically a device according to the invention and comprising reference paths.

FIG. 6 shows diagrammatically a device according to the invention and, operating in polarized light.

FIG. 7 is a diagram of a processing circuit.

Referring now to FIG. 1, at both ends of an atmospheric path which has to be studied, there are provided two optico-electronic units essentially composed of transmitter-receiver members la, 5a and lb, 5b which will be described farther on. The two radiation transmitters la, 1b are respectively fed by power gener ators 2a, 2b and are placed behind convergent optics 3a, 3b. Convergent mirrors 4a, 4b each receive a given part of the radiation transmitted by the source of the opposite unit. These mirrors reflect the respective radiation on photosensitive receivers: 5a, 5b. The mirrors 4a, 4b and optics'3a, 3b are so oriented so that the optic axes coincide, which leads to obtaining a maximum connection between the source la and the receiver 5b on the one hand, and the source lb and the receiver 5a on the other hand. There are then two beams of radiation passing through the atmosphere to be studied, a first beam formed of rays such as 60 and a second beam, directed in an inverse manner, being formed of rays such as 6b.

Devices 70, 7b form reference optic paths which are short and stable, between the source la and the re ceiver 5a and respectively, between the source lb and the receiver 5b. Such devices can be made of combinations of flat mirrors, or more advantageously, as shown by means of bunches of glass threads called optic fibres." Thus, the beam 7a, respectively 7b, send part of the radiations transmitted by the transmitter la, respectively lb, directly on to their associated receiver 5a, 5b or by means of a spherical mirror 4a, respectively 4b.

The receivers 50, 5b are respectively connected to electric signal amplifier devices 80, 8b whose outputs are connected to separator means 9a, 10a and 9b, 10b. Said separator means are described farther on and have the purpose of distinguishing, among the signals supplied by each of the two receivers 5a and 5b, those due to adjacent associated sources (10 for 5a, 1b for 5b) and those due to distant sources (la for 5b, lb for 5a).

At the output of the separator means, there are thus four different electric signals available on four conductors 11, 12, 13 and 14.

One designates by E, and E the powers radiated respectively by the sources 1a and lb.

0, and the respective transmissions coefficients of the optics 3a and 3b.

M, and M the respective retransmission coefficients of the mirrors 4a and 4b.

S, S the respective sensitiveness of the receivers 5a and 5b.

R, and R the respective transmission coefficients of theeference devices 7a 7b.

G, and G, the respective gains of the amplifiers 8a and 8b.

K the transmissive power of the atmosphere for the atmospheric path under consideration.

A, B, C and D the values of the electric signals on the respective conductors 11, 12, 13, 14.

It is also assumed that the separator means 90, 9b, 10a, 10b do not introduce any appreciable change in the electric signals which are transmitted. This is rather conveniently the case when electric filters are used as separator means.

The various signals A, B, C and D have then the following values From these signal values, it is possible to extract either by computation, or by means of an electronic circuit 15, or processing unit, an electric value independent of the values E,,E,, 8,, 8,, G,, G, and depending only on the transmissive power of the atmosphere K and constant geometric characteristics of the device. An embodiment of a processing circuit which is particularly appropriate is described hereafter with reference to FIG. 7.

Actually, if one effects the products A X D and B X C, and then the quotient B X C/A X D, one obtains the following expression which is simplified down to B X 7 X D 1 2 M1 z/ i 2) If the geometrical characteristics of the installation are constant, the term placed in front of K in the second member of the preceding equation is a constant, and we obtain B X C/A X D (constant) X K 4 One thus provides at the output of the processing circuit 15, an appliancesensitive to said electric value, for instance, a numerical counter 16 which thus measures the transmissive power of the atmosphere.

In a particular embodiments of the invention, each of the two transmitter-receiver units comprising the optico-electronic means of FIG. 1, is made, as shown in FIG. 2, ofa metal tube 21 which, at one of its ends, supports a lens 3 of the convergent type and in the focus of said lens an electro-luminescent diode 1 is mounted. The other end of the metal tube 21 comprises a filter 22 protecting the photosensitive receiver 5 against radiations of a wave length differing from those composing the transmission spectrum of the electroluminescent diode l.

The receiver 5 is a photodiode or 2 phototransistor, fixed on the axis of the tube so as to be at the converging focus of the parabolic mirror 4. The feed circuit of the diodes 1 and 5 is shown diagrammatically at 1 and 5 respectively.

An optic fiber 23 is engaged by its ends in two openings made in the tube 21, so as to send from the electroluminescent diode 1 towards the phototransistor 5, a slight and constant fraction of the radiation transmitted.

The mirror 4 and tube 21 unit provided with members 1, 3, 5, 22 and 23, and held by a support 24, is installed in a tubular casing 25 whose length gives protection against direct sun rays as well as against detrimental weather.

In a particular embodiment of the invention, the electroluminescent diodes of each of the two opticoelectronic units are energized by generators of electric current with a different modulation frequency. Each of these generators, designated by 2a, 2b in FIG. 1', is composed of an oscillator connected to an electric amplifier. The respective frequencies F, and F, of the two oscillators being different, this enables very easily to distinguish, by means of the means 9a, 9b, 10a and 10b of FIG. 1, which are simple filters respectively tuned on the modulation frequencies F, and F,, the signals coming either from the associated electroluminescent diode, or from the distant electroluminescent diode. These different signals are thus applied to the processing unit or data processing circuit 15 through wires 1 l14, as previously explained.

In another particular embodiment of the invention shown in FIG. 3, the electroluminescent diodes 1a and lb are fed by pulse signals. The duration of the pulses is less than the time taken by light for passing through the atmospheric path studied. In FIG. 3, where the same members as those of FIGS. 1 and 2 bear the same reference numerals, a pulse generator 16 simultaneously feeds electric power amplifiers 2a and 2b connected to the electroluminescent diodes Ia and lb. The pulses E,, E which are then radiated by the diodes la and 1b, are shown on FIG. 4, lines a and d.

The current pulse E, S, delivered by the receiver 50 under the effect of the radiation from diode 1a, is shown on line b. It is practically simultaneous with the pulse E,, line a. On the contrary, the current. pulse delivered by the receiver 5b, under the influence of the diode 1a, is offset of the time taken by the light to traverse the atmospheric path as shown at E, S line 0.

The lines e and f show that the same takes place with regard to the pulses transmitted by the electroluminescent diode lb.

Each of the receivers 5a, 5b thus successively delivers two pulses, shown by E, S, (line b) and E S (linef) for the receiver 5a and by E 8, (line e) and E, S (line for the receiver b. The separation of these signals is made by two groups of two electronic switches or gate circuits, for instance, the circuits AND 19a, 20a, 19b, 20b, which are piloted by the pulse generator 16 in such a manner that the four signals A, B, C, D of the group of expressions (1) appear each individually on the conductors l1, 12, 13, 14 and are applied to the data processing circuit 15.

Another arrangement, based on the process initially described, has the object of simplifying said process with a view of obtaining a signal directly proportional to the transmissive power, while having still the advantages of being independent of emissivity variations of the sources as well as sensitivity variations of the receivers.

The process, according to this arrangement, is modified in that one of the local transmitters is formed by a reference transmitter optically coupled with each of the receivers by reference optic paths.

A device for operating this modified process comprises, at one end of the measurement path, an opticoelectronic transmitter-receiver, a receiver at the other end, and a reference unit formed by a reference transmitter optically coupled by reference paths with each of said receivers, as well as a circuit for carrying out the ratio of the products of homologous signals from each of these receivers.

This device affords the advantage of preserving to the measurement path in the atmosphere, the maximum value provided by the qualities of the equipment. It is to be noted that the measurement remains then independent of gain variations of the electric amplifiers generally associated with the photo-sensitive receivers. Moreover, the measurements and reference paths can be entirely independent and isolated, thus avoiding the necessity of eventual protection against intergering retrodiffusions.

in this device however, it is not possible, in the event of a defect of a member such as light source, photosensitive receiver or amplifier, to be provided with still a measurement even of less accuracy, whereas this possibility exists when one uses a symetrical double channel wherein one of them still supplies a signal.

Said device is shown in FIG. 5 wherein at one end of the atmospheric path studied, is shown a transmitter receiver essentially composed by a radiation transmitter 31a fed by a power generator 32a at a given frequency F, and a photo-sensitive receiver 35a. These various members have been described in connection with FIG. 1. The transmitter 310 is placed behind a convergent optic 33a in order to energize, through the measurement optic path and by means of a convergent mirror the receiver 35b placed at the other end. The optic 33a and mirror 34b are so mounted that their optical axes coincide, thus affordinga maximum coupling between the source 310 and the receiver 35b.

7 A fraction of the radiations transmitted by the source 31a is directed to the associated receiver 35a by a stable optic path 37a formed, for instance, by optic fibres 6 and making a constant optic coupling between the source 31a and the receiver 35a.

A source of reference radiations 37 is fed by a power source 32 at a given frequency F, and simultaneously 5 irradiates respectively through stable optic paths or reference paths 37 and 37b, the receivers 35a and 35b, The stable optic paths 37a, 37 and 37b can be made in the form of combinations of flat mirrors connected by enclosures preserved from the atmosphere whose transmissive power is to be measured, or more advantageously, in the form of optic fibres as described above.

The electric circuit is made in a similar manner to that shown in FIG. 1, by means of amplifiers 38a, 38b, and separator means 39a, 40a and 39b, 40b, electric filters for instance, that have the purpose of distinguishing, among the signals supplied by each of the receivers 35a and 35b, those due to the source 31a and those due to the reference source 31.

Thus, four different electric signals A, B, C, D provided at the output of this circuit unit are present on four conductors 41, 42, 43, 44.

One designates by E, and E the powers respectively radiated by the source 31a and reference source 31 at respective frequencies F, and F 0 the transmission coefficient of the optic 330;

M the retransmission coefficient of the mirror 34b;

S, and S, the respective sensitivenesses of the receivers 35a and 35b;

R, R and R the respective transmission coeffi' cients of the reference devices 37a, 37 and 37b; G, and G the respective gains of the amplifiers 38a and 38b; K the transmissive power of the atmosphere for the atmospheric path studied;

If, in a particular circuit 45, similar to the data processing circuit 15, we carry out the ratio of the products B X C/A X D, we obtain the following expression BX CIA X D=E R: S, G, X E, OMS, K (i /E, R, S, G, X E, R, S, (I,

which may be written B X C/A X D (R, O M/R, R K (7) Now, the various factors of the term which precede K are independent of the transmissive power of the atmosphere, variations of source characteristics, photosensitive receivers and amplifiers. If, by construction, the geometrical characteristics of the various members 33a, 34b, 37a, 37 and 37b are constant, we can put down B X C/A X D (constant) X K The circuit 45 thus supplies an information directly proportional to the transmissive power K, which is measured by any suitable device, for instance, a counter 46 connected to the output of the data processing circuit 45.

In a specific form of embodiment, the source 31a, associated with a lens of photographic converging type 33a, is an electroluminescent diode placed at the focus of this lens. The receiver 35b is a photodiode or phototransistor placed in the focus of the parabolic or spherical mirror 34b. In the casing containing the source 31a and lens 33a there is also a photodiode or phototransistor 35a on whose input face are applied the ends of two flexible light conductors, such as optic fibres 37a and 37 which respectively collect the light from the source 310 and that from the reference source 31 which is also an electroluminescent diode. On the output face of the electro-luminescent diode 31 in addition to the light conductor 37 there is also applied one end of another light conductor 37b of which the other end is provided to light up the receiver 35b.

In this specific embodiment, the electroluminescent diodes 31a and 31, are energized by different modulation frequency electric current generators, as already described with reference to FIG. 1. It has been shown that by means of simple filters 39a, 40a and 39b, 40b, tuned on one of the two modulation frequencies, it was possible to separate the respective signals coming from radiations of the diode 31a, and of the diode 31.

It is quite obvious that the impulsional process described with reference toFIG. 3 is applicable for distributing and collecting in time the signals relating to the diodes 31a and 31, which are similar to the diodes 1a and 1b of said FIG. 3. The positions of the members 31 and 35a, relatively to the source 310 and to the receiver 35b, as well as the arrangement and length of the optic fibres which arise out of these relative positions can be modified without entailing a change in the working of the device. Preferably, one chooses, for the reference source 31, such location that the dimming of the light signals in the channels 37 and 37b is substantially the same (R, R

Retrodiffusions interfering with the light on fog particles in suspension in the atmosphere close to the photosensitive receivers, introduce an error factor whose eventual disturbing influence is only partially eliminated in the preceding arrangement on the reference paths.

A supplementary arrangement enables to cancel, to a large extent, the disturbing influence of retrodiffusion on the accuracy of the device of the invention.

According to this arrangement, the light signals transmitted by each transmitter towards the distant receiver are polarized according to different respective directions, preferably orthogonal, and said polarized signals are received by the receiver through an analyzer whose polarization plane is parallel to the polarization plane of the light transmitted from the distant transmitter.

In FIG. 6, which shows some of the members of FIG. 1, each of the radiation sources la, 1b, is provided, on

the transmission path towards the remote receiver 5a, 5b, with a polarizing screen 51a, 51b having the known property of polarizing the light according to a preferred direction. The direction chosen for the polarizer 51a is preferably, orthogonal to the direction chosen for the polarizer 51b. This is diagrammatized in the figure by an arrow symbolizing the polarization plane, said arrow being placed inside a circle.

Correlatively, each of the photo-sensitive receivers 50, 5b, is preceded, on the reception path of the remote transmitter, with a screen for analyzing the polarized light, said screen having the known property of dimming the polarized light, except in a preferred direction. This is also diagrammatized in the figure by an arrow, symbolizing the polarization plane, said arrow being placed inside a circle.

The polarizing direction chosen by the analyzer 52a relating to the receiver 5a is thus parallel to the polarization direction of the polarizer 51b associated with the transmitter 1b, and hence orthogonal to the polarization direction of the polarizer 51a associated with the transmitter 1. In the very same way, the analyzer 52b associated with the receiver 5b has a polarization direction parallel to that of the polarizer 51a of the transmitter 1a and orthogonal to that of the polarizer 51b of the transmitter lb.

The reference optic paths made through the devices 7a, 7b and intended to provide the automatic compensation of the variations in the characteristics of the various members are not concerned with the above polarizers and analyzers, and the transmission on their path takes place with a non-polarized light.

By referring to the designation previously used and by incorporating the respective dimmin gs 71 and 72 on the optic paths 6a and 6b, due to the presence of polarizers and analyzers, and moreover, by taking into account the interfering retrodifi'usion signals of the light E and F, symbolized in the figure by the arrows 12 and 14 and provided with a coefficient x (K) depending on the transparency characteristics of the atmosphere and also with dimming coefficients F1 and F2 due to the effect of cossed couples of polarizers and analyzers, 51a, 52a and 51b, 52b, one obtains for the signals A, B, C, D previously defined, and for E, F, the following values A =E, R, S, G,

B=E, S, 0,M, KG, 7,

C=E, S, 0, M, KG, y,

D E, R, S, G,

E=E, X (K) I, S, G,

F=E, X (K) I, S, G,

The modified ratio BXC/(A+E)' (D+F) has for expression after simplifying The geometrical characteristics being supposed to remain constant, this expression can be put down as follows, after making the generally accepted approximations on the terms of the denominator Thus, the error brought by retrodiffusion on the mea suring of K and which is normally figured by the ratios x(K)/R, and .r(K)/R is multiplied by the dimming coefficients I, and I, introduced by using polarized light.

The value of the terms R, and R which characterize the conveyance of the light on the reference paths 7a, 7b can be increased to the point of making negligible the influence of the variations of x(K). Nevertheless, for avoiding too great an unbalance between the energies received by the photosensitive receiver 50 by the path 70 and the path 6b, as well as between the energies that the receiver b receives by the path 7b and the path 6a, it is necessary to substantially have cl R, =0 M 1 Y1 These conditions impose an upper limit at R, and R It is then very advantageous to reduce the maximum error by utilizing the dimmings I and F which may, for instance, be of 0.1.

Thus, as it has been explained above, the signals E and F, coming from the interfering retrodiffusion of the light, respectively added to the signals A and B coming from the reference path, since they are of the same modulation frequency (continuous as in FIG. 1 or im- 'pulsional as in FIG. 3). This means that the processing of the signals obtained, according to FIG. 6, is identical to that obtained according to FIG. 1 or FIG. 3, the signal A of said figures then being replaced by A E and the signal D by D F to be processed by an electronic circuit analogous to the circuit or 45 of FIGS. 1 and 3.

It should be noticed that without going outside the scope of the invention, the positions of the electroluminescent diodes and photo-transistors could be inverted, the colour filters and photo-transistors then being placed behind the lenses and the electroluminescent diodes in the focus of the parabolic mirrors. Likewise, the electroluminescent diodes can be replaced by light or radiative sources of different types, such as flash lamps, coherent light transmitters, filament incandescent lamps, fluorescent gas lamps.

FIG. 7 shows an advantageous embodiment of the processing device designated by reference number 15 in FIGS. 1 and 3 and by 45 in FIG. 5. In said FIG. 7, reference letters A, B, C, D designate the four output magnitudes, under the form of electric voltage, for instance, whose product of the ratios B/A C/D is to be made.

The dividend voltage B is applied on the input 69 of a two-input differential amplifier 61 whose second input is connected by a connection 70, comprising a resistance 68, to the output of a numerical-to-analogue converter 66. A gate circuit 62, a flip-flop, for instance, can be released by a signal applied on the input 72 and re-locked by the output signal of the amplifier 61 applied to the input 71. The gate circuit 62 is placed between a pulse generator 63 and a pulse counter 65 of a N capacity, the whole forming, with the members 61 and 66, an analogue-to-numerical converter 64. The

counter is reset by a signal applied on one of its connections 73.

The stages of the counter 65 are individually connected to the numerical-to-analogrue converter 66. A network of balanced resistances 67a-67e is connected to a connection 74 on which is applied the divider voltage A so that each increase of one unit of the contents of the counter 65 causes an increase of a constant value of the output voltage existing on the connection 70. As the voltage A is applied on the network 67a 67c, the value of these steps is proportional to this signal A. Furthermore, the circuit is dimensioned so that when the counter reaches its full capacity N, the voltage on is then equal to the voltage A applied on 74. The whole unit 61 to 74 thus forms a first elementary divider circurt.

A shunt connected to the output of the circuit 62 transmits the pulses therefrom on to a second elementary divider circuit 61' to 74' similar to the firstelementary divider circuit 61 to 74, on which are respectively applied through connections 74 and 69', the dividend voltage C and the divider voltage D of the second C/D term of the product to be made.

One designates by M the capacity of the counter 65. The shunt 75 leads, on the one hand, by a connection 72' on the release control of the gate circuit 62', and on the other hand, by a connection 73' on the resetting of the counter 65'. The output of the pulses is collected on a connection 75' and can be connected to a utilization device, such as a pulse counter 76 analogous to the counters 46 and 16 of the preceding figures.

Working of this device will be now explained. An external signal is first applied on one hand on the connection 73 to reset the counter 65, and on the other hand on the connection 72 to release the circuit 62. The counter 65 then being at zero, the output of the converter 66 is nil.

The pulses from the generator 63 then reach the counter 65 which begins to totalize them. Correlatively, the output voltage of the converter 66 increases by one step for each pulse received by the counter 65, and this output voltage is compared in the amplifier 61 with the input voltage B. When this output voltage reaches the value of the voltage B, this equality isrevealed by the amplifier 61 which supplies, on the connection 71, a signal causing the locking of the circuit 62 and interrupting the arrival of pulses from the generator 63 in the counter 65.

If, at this moment, the contents of the counter 65 has reached a value n and, since the capacity thereof is equal to N, the value N corresponds to the value A, and one may put down n/B N/A hence n B/A N.

The number n is thus proportional to the quotient BIA.

Each of the pulses n transmitted on the second ele mentary divider 61 74' plays the same part as the ex temal signal applied on the connections 72 and 73 of the first elementary divider. In this second divider, the counter 65' receives, for each of the n pulses arriving on 75, a number m of pulses, so that m M (C/D).

During the complete working cycle defined by the speed of the external signals periodically sent on the connections 72 and 73, the unit supplies on the output 75 a number of pulses n -m=N-M'( N and M being the constants, one actually collects a number of pulses proportional to the product of the, two quotients (8/11) X (C/D).

Nevertheless, it should be noted that the choice of frequencies of the pulse generators 63 ,'63 is not totally independent. Actually, seeing that the time between the pulses of the generator 63 must eventually contain M pulses from the generator 63', the latter generator must have a frequency at least M times greater than that of the generator 63.

It is obvious that one might connect a third elementary divider circuit in tandem and pilot it by the output pulses appearing on the connection 75 in like manner to the second divider which is piloted by the pulses arriving on the connection 75, the generator frequency of this third divider complying with the condition previous mentioned, and so on. By thus connecting a number of elementary dividers on which are respectively applied analogous signals of dividend and divider of a number corresponding of quotients, one finally obtains a number of output pulses proportional to the product of all these quotients.

We claim: 1. Device for determining the transparency of the atmosphere between two places fixing a measurement path, comprising a first light transmitter arranged at one of these two places for sending a first main light signal to the other place through the atmosphere of which the transparency is to be determined, a first light receiver arranged at the said other place for receiving the first main light signal and producing a first electrical signal, a second light transmitter for sending a second main light signal, different from the first main light signal, a second light receiver for receiving the second light signal and producing a second electrical signal, light conductors isolated from the atmosphere to be studied and optically connecting the first light transmitter and the second light receiver for sending to the latter a first reference light signal, whereby said second electrical signal contains a data representative of the second main light signal and a data representative of the first reference light signal,

the second light transmitter and the first light receiver for sending to the latter a second reference light signal, whereby the first electrical signal contains a data representative of the first main light signal and a data representative of the second reference light signal,

a first separating means for differentiating in said second electrical signal the data representative of the second main light signal from the data representative of the first reference light signal,

a second separating means for differentiating in said first electrical signal the data representative of the first main light signal from the data representative of the second reference light signal, and a data processing circuit connected to said first and second separating means at output thereof, said data processing circuit comprising means for multiplying the data representative of the second main light signal times the data representative of the first main light signal whereby is obtained a first product means for multiplying the data representative of the first reference light signal times the data representative of the second reference light signal whereby obtaining a second product, and

means for dividing the first product by the second product whereby obtaining an output signal which is representative of the transparency of the atmosphere to be determined.

2. Device according to claim 1, comprising a first optical system connected to the first light transmitter for sending the first main light signal to the first light receiver through the atmosphere to be studied, and

a second optical system connected to the second light transmitter for sending the second main light signal to the second light receiver through the atmoshpere to be studied.

3. Device according to claim 1, comprising an optical system connected to the first light transmitter for sending the first main light signal to the first light receiver through the atmosphere to be studied, and

a light conductor isolated from the atmosphere to be studied and optically connecting the second light transmitter to the second light receiver for sending to the latter the second main light signal.

4. Device according to claim 2, additionally comprisa first polarizer screen inserted, at the output of the first light transmitter, on the path of the first main light signal, and a first analyzer, screen preceding the first light receiver on the path of the first main light signal, the polarizing directions of the said polarizer and analyzer being parallel,

a second polarizer screen inserted, at the output of the second light transmitter, on the path of the second main light signal, and a second analyzer screen, preceding the second light receiver on the path of the second main light signal, the polarizing directions of the said polarizer and analyzer being parallel but orthogonal to the polarizing directions of the first polarizer and analyzer screens.

5. Device according to claim 1 further comprising a modulated power generator connected to the first light transmitter for feeding it at a first frequency and a modulated power generator connected to the second transmitter for feeding it at a second frequency, different from the first one,

a filtering circuit tuned to the first frequency and a filtering circuit tuned to the second frequency both connected to the output of the second receiver and respectively providing the data representative of the second main light signal and the data representative of the first reference light signal,

a filtering circuit tuned to the first frequency and a filtering circuit tuned to the second frequency both connected to the output of the first receiver and respectively providing the data representative of the first main light signal and the data representative of the second refeence light signal.

6. Device according to claim 1 further comprising an electric power pulse generator connected to the two light transmitters, this generator having a period of duration less than the time required for the light to go from one place to the other and four gate circuits which are piloted by the pulse generator and connected the first and second ones, to the output of the first light receiver for producing respectively the data representative of the first reference light signal and the data representative of the second main light signal,

the third and fourth ones, to the output of the second light receiver for producing respectively the data representative of the first main light signal and the data representative of the second reference light signal.

7. Device according to claim 1 in which the data processing circuit comprises a first pulse generator,

a first pulse counter,

a first gate circuit inserted between the generator and the counter,

a first scale step numerical-to-analog converter receiving the data representative of the first reference light signal and connected to the first counter for producing a scale step" output signal increasing of one step when the content of the counter increases of one unit, every step being proportional to the data representative of the first reference light signal, i

a first comparison circuit receiving the data representative of the second main light signal and the output signal of the first converter and connected to the first gate circuit for causing its locking when the two received signals are equal,

a second pulse generator,

a second pulse counter, having a given capacity,

a second gate circuit inserted between the generator and the counter, the releasing input of which being connected to the output of the first gate circuit,

a second scale step numerical-to-analog converter receiving the data representative of the second reference light signal and associated to the second counter for producing a scale step output signal increasing of one step when the content of the counter increases of one unit every step being proportional to the data representative of the second reference light signal,

a second comparison circuit receiving the data representative of the first main light signal and the output signal of the second converter and connected to the second gate circuit for causing its locking' greater than the frequency of the first pulse generator. 

1. Device for determining the transparency of the atmosphere between two places fixing a measurement path, comprising a first light transmitter arranged at one of these two places for sending a first main light signal to the other place through the atmosphere of which the transparency is to be determined, a first light receiver arranged at the said other place for receiving the first main light signal and producing a first electrical signal, a second light transmitter for sending a second main light signal, different from the first main light signal, a second light receiver for receiving the second light signal and producing a second electrical signal, light conductors isolated from the atmosphere to be studied and optically connecting : the first light transmitter and the second light receiver for sending to the latter a first reference light signal, whereby said second electrical signal contains a data representative of the second main light signal and a data representative of the first reference light signal, the second light transmitter and the first light receiver for sending to the latter a second reference light signal, whereby the first electrical signal contains a data representative of the first main light signal and a data representative of the second reference light signal, a first separating means for differentiating in said second electrical signal the data representative of the second main light signal from the data representative of the first reference light signal, a second separating means for differentiating in said first electrical signal the data representative of the first main light signal from the data representative of the second reference light signal, and a data processing circuit connected to said first and second separating means at output thereof, said data processing circuit comprising : means fOr multiplying the data representative of the second main light signal times the data representative of the first main light signal whereby is obtained a first product , means for multiplying the data representative of the first reference light signal times the data representative of the second reference light signal whereby obtaining a second product, and means for dividing the first product by the second product whereby obtaining an output signal which is representative of the transparency of the atmosphere to be determined.
 2. Device according to claim 1, comprising : a first optical system connected to the first light transmitter for sending the first main light signal to the first light receiver through the atmosphere to be studied, and a second optical system connected to the second light transmitter for sending the second main light signal to the second light receiver through the atmoshpere to be studied.
 3. Device according to claim 1, comprising : an optical system connected to the first light transmitter for sending the first main light signal to the first light receiver through the atmosphere to be studied, and a light conductor isolated from the atmosphere to be studied and optically connecting the second light transmitter to the second light receiver for sending to the latter the second main light signal.
 4. Device according to claim 2, additionally comprising: a first polarizer screen inserted, at the output of the first light transmitter, on the path of the first main light signal, and a first analyzer, screen preceding the first light receiver on the path of the first main light signal, the polarizing directions of the said polarizer and analyzer being parallel, a second polarizer screen inserted, at the output of the second light transmitter, on the path of the second main light signal, and a second analyzer screen, preceding the second light receiver on the path of the second main light signal, the polarizing directions of the said polarizer and analyzer being parallel but orthogonal to the polarizing directions of the first polarizer and analyzer screens.
 5. Device according to claim 1 further comprising : a modulated power generator connected to the first light transmitter for feeding it at a first frequency and a modulated power generator connected to the second transmitter for feeding it at a second frequency, different from the first one, a filtering circuit tuned to the first frequency and a filtering circuit tuned to the second frequency both connected to the output of the second receiver and respectively providing the data representative of the second main light signal and the data representative of the first reference light signal, a filtering circuit tuned to the first frequency and a filtering circuit tuned to the second frequency both connected to the output of the first receiver and respectively providing the data representative of the first main light signal and the data representative of the second refeence light signal.
 6. Device according to claim 1 further comprising an electric power pulse generator connected to the two light transmitters, this generator having a period of duration less than the time required for the light to go from one place to the other and four gate circuits which are piloted by the pulse generator and connected : the first and second ones, to the output of the first light receiver for producing respectively the data representative of the first reference light signal and the data representative of the second main light signal, the third and fourth ones, to the output of the second light receiver for producing respectively the data representative of the first main light signal and the data representative of the second reference light signal.
 7. Device according to claim 1 in which the data processing circuit comprises : a first pulse generator, a first pulse counter, a first gate circuit inserted between the generator and the couNter, a first ''''scale step'''' numerical-to-analog converter receiving the data representative of the first reference light signal and connected to the first counter for producing a ''''scale step'''' output signal increasing of one step when the content of the counter increases of one unit, every step being proportional to the data representative of the first reference light signal, a first comparison circuit receiving the data representative of the second main light signal and the output signal of the first converter and connected to the first gate circuit for causing its locking when the two received signals are equal, a second pulse generator, a second pulse counter, having a given capacity, a second gate circuit inserted between the generator and the counter, the releasing input of which being connected to the output of the first gate circuit, a second ''''scale step'''' numerical-to-analog converter receiving the data representative of the second reference light signal and associated to the second counter for producing a ''''scale step'''' output signal increasing of one step when the content of the counter increases of one unit every step being proportional to the data representative of the second reference light signal, a second comparison circuit receiving the data representative of the first main light signal and the output signal of the second converter and connected to the second gate circuit for causing its locking when the two received signals are equal, and a pulse counter connected to the output of the second gate circuit.
 8. Device according to claim 7, in which the frequency of the second pulse generator is at least M times greater than the frequency of the first pulse generator. 